The present invention relates to electric meters and, more particularly, to rotor disks in electric meters for measuring the consumption of electric energy.
Conventional electro-mechanical electric meters employ a conductive metal disk rotated as the rotor of a small induction motor by interaction with fluxes generated by opposed potential and current stators. When the fluxes produced by the current and potential stators are in quadrature, the rotational torque experienced by the disk is proportional to the voltage applied to the load multiplied by the current consumed by the load: that is, the power consumed by the load. Disk rotation is magnetically resisted in proportion to its rotational speed. Thus, the disk speed is proportional to the power consumed by the load. Each rotation of the disk represents a predetermined increment of energy consumed. The rotations of the disk are accumulated over time in a mechanical or electronic accumulator, or register, for billing purposes by the utility supplying the power.
The torque on the disk is produced by eddy currents induced in the disk which interact with the magnetic fields produced by the potential and current stators. A single-phase meter, employing a single potential stator and a single current stator, is capable of using a solid disk of a highly conductive material such as, for example, aluminum. The eddy currents describe expanding circles about poles of the potential and current stators until the eddy currents are blocked by, for example, reaching the edge of the disk after which the circles are distorted into cardioids. The radial components of the eddy currents contribute to rotational torque. The tangential components of eddy currents produce only radial forces which are resisted by conventional bearings.
Problems arise when two or three sets of potential and current stators are required to measure power consumed in a poly-phase system. If two or more sets of potential and current stators are mounted to drive a single solid aluminum disk, the eddy currents circulating about the location of one set of potential and current stators are capable of reacting with the flux about the location of another set of potential and current stators. This reaction between the eddy currents from one source with the flux from a neighboring source may be positive, i.e. contributing to forward disk rotation, or negative, i.e. retarding forward disk rotation. Thus, two or more flux sources on a solid disk are capable of interacting in a way which degrades measurement precision.
One solution for overcoming the interference problem includes employing separate aluminum disks spaced apart concentrically on a single shaft, one for each set of potential and current stators. ln this way, the eddy currents in one disk produced by one set of potential and current stators are incapable of reaching any other disk to interfere with the flux from any other set of potential and current stators. This solution is undesirable because of the substantial extra cost and the greatly increased envelope dimensions required.
A further solution, and the one most commonly employed in commercial polyphase meters, employs a laminated disk having, for example about seven mutually insulated metallic layers bonded together. Two or three sets of potential and current stators are angularly spaced apart about the circumference of the laminated disk. Each of the metallic layers includes radial slots extending from their perimeters almost to their centers to separate each layer effectively into a set of independent pie-shaped segments. The angular extent of the pie-shaped segments is selected to block eddy currents originating in one set of potential and current stators from reaching a region where they can interact with the fluxes from any other set of potential and current stators. The fabrication of a laminated disk is expensive and the reduced areas over which eddy currents can flow as a result of interruption by the slots reduces the torque.
A still further solution employs a metallic rotor disk from which an annular portion is removed to form an inner disk and an outer annulus. The removed annulus is filled with a non-conducting material. Process control in maintaining alignment between the inner and outer metallic portions is difficult. ln addition, the non-conducting filling the annulus is fragile and makes it difficult to straighten warped portions of the metallic parts which often occur as a result of the process of forming the non-conducting material.